Power supply equipment for motor vehicle

ABSTRACT

Power supply equipment for a motor vehicle, comprising a motor generator, an inverter for driving said motor generator, a battery and a capacitor of an electrical double layer, wherein the capacitor is directly connected to a DC side of the inverter and the battery is connected in parallel with the capacitor via a first switching unit. When an engine is started up, the power stored in the capacitor is used, and the switching units are turned off to separate the battery from the start-up of the engine.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to power supply equipment for motorvehicles, including a motor generator driven by an inverter, and moreparticularly to power supply equipment for a motor vehicle, suitable forincreasing the life of a battery, using an electrical double layercapacitor.

[0002] Recently, a system has been developed, which has a single motorgenerator that is used both as a conventional starter and a conventionalalternator in which 42 volts are employed as a power supply voltage inorder to improve a fuel efficiency in the vehicle. The motor generatoris ordinarily driven by an inverter, and has an idle stopping functionthat stops the engine when the vehicle stops and that re-starts theengine when the vehicle is driven, a regenerative braking function thatcharges the battery with decelerating energy by a generator operationperformed when the vehicle is decelerated, and an assisting functionthat applies a drive torque to a driving shaft in acceleration.

[0003] Application of a lead acid battery chargeable/dischargeable witha large electrical current at low cost has been studied as a battery forpower supply. Since the output of this battery can lower at lowtemperatures/low charge rates, the engine is difficult to start up.Thus, it is necessary for the battery to have a sufficient capacity.Charging/discharging the battery with a large current affects the lifeof the battery adversely, and it is necessary to restrict the chargingcurrent especially.

[0004] In view of these problems, power supply equipment for motorvehicles has been studied that is intended to increase the service lifeof the battery by using a capacitor of an electrical double layer havinga long life compared to the battery and being chargeable/dischargeablewith a large current.

[0005] A first example is JP-A-10-184506 and 10-191576, which disclosepower supply equipment for a motor vehicle, comprising a capacitor of anelectrical double layer and a battery connected through a diode, a relayswitch and a resistor in order to start up the engine with power storedin the capacitor.

[0006] A second example is JP-A-4-271209 that discloses power supplyequipment for a motor vehicle, in which a capacitor of an electricaldouble layer and a battery are connected in parallel and also connectedto corresponding switches in series in order to adjust quantities ofenergy stored in the capacitor and the battery, respectively, bycontrolling the operations of the switches.

[0007] A third example is JP-A-2000-156919 that discloses power supplyequipment for a motor vehicle, in which an inverter is connected at itsoutput to a capacitor of an electrical double layer, which is thenconnected to a battery through a DC/DC converter.

[0008] In the first example, however, since it is only possible toeither charge the capacitor or the battery during the chargingoperation, the capacitor cannot increase the battery life by absorbinglarge transitional current.

[0009] In the second example, the capacitor cannot absorb a ripplecurrent due to the switching of the inverter when the switch of thecapacitor is off, and a capacity of an filter capacitor of the invertercannot be reduced. Thus, it is impossible to achieve sufficient costreduction.

[0010] In the third example, an increase in the cost cannot be avoideddue to the employment of the DC/DC converter.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the present invention to providelong-life, low-cost power supply equipment for motor vehicles.

[0012] (1) In order to achieve the above object, the present inventionprovides power supply equipment for a motor vehicle, comprising a motorgenerator, an inverter for driving the motor generator, a battery and acapacitor of an electrical double layer, wherein the capacitor isdirectly connected to a DC side of the inverter and the battery isconnected in parallel with the capacitor via first switching means. Suchcomposition gives the equipment an increased life and reduces its cost.

[0013] (2) Preferably, the power supply equipment further comprisescontrol means for turning off the first switching means in the start-upof an engine to separate the battery from the capacitor and for turningon the first switching means after the start-up of the engine to connectthe battery to the capacitor.

[0014] (3) Preferably, the power supply equipment defined in (2) furthercomprises: a series circuit of a resistor and second switching meansbeing connected in parallel with the first switching means.

[0015] (4) Preferably, in the power supply equipment defined in (3) thecontrol means determines which of the first and second switching meansshould be turned on, depending on a difference in voltage between thecapacitor and the battery when the battery and the capacitor areconnected by the first or second switching means.

[0016] (5) Preferably, the power supply equipment defined in (1) furthercomprises: a second capacitor connected in parallel with thefirst-mentioned capacitor between the inverter and the capacitor foreliminating high frequency ripples.

[0017] (6) Preferably, in the power supply equipment defined in (1) thefirst switching means comprises a diode conductible at all times so asto allow an electrical current to flow from the inverter to the battery.

[0018] (7) Preferably, in the power supply equipment defined in (1) thefirst switching means comprises a diode conductible at all times so asto allow an electrical current to flow from the battery to the inverter.

[0019] (8) In order to achieve the above object, power supply equipmentfor a motor vehicle, comprising a motor generator, an inverter fordriving the motor generator, a battery and a capacitor of an electricaldouble layer, wherein the battery has a plurality of different (higherand lower) voltage terminals; the capacitor being directly connected toa DC side of the inverter; the capacitor being connected on its highervoltage side to a higher voltage terminal of the battery via firstswitching means; and the capacitor being connected on its higher voltageside to a lower voltage terminal of the battery via second switchingmeans.

[0020] Such composition gives the equipment an increased life andreduces its cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a block diagram of power supply equipment for a motorvehicle according to a first embodiment of the present invention.

[0022]FIG. 2 is a flowchart of a control process performed by a powersupply controller of the power supply equipment for the motor vehicle asthe first embodiment.

[0023]FIG. 3 is a block diagram of power supply equipment for a motorvehicle according to a second embodiment of the present invention.

[0024]FIG. 4 is a block diagram of power supply equipment for a motorvehicle according to a third embodiment of the present invention.

[0025]FIG. 5 is a block diagram of power supply equipment for a motorvehicle according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Referring to FIGS. 1 and 2, the composition of the power supplyequipment for a motor vehicle according to the first embodiment of thepresent invention will be described next.

[0027]FIG. 1 is a block diagram of the power supply equipment for amotor vehicle according to the first embodiment of the presentinvention. FIG. 2 is a flowchart of the control process performed by thepower supply equipment for a motor vehicle according to the firstembodiment. FIG. 1 shows a 42 V-power supply system for a motor vehicleto which the embodiment of the present invention is applied.

[0028] As shown in FIG. 1, a rotational shaft of the engine 1 isconnected to a motor generator 2. The motor generator 2 operates as amotor to start up/assist in accelerating the engine 1. The motorgenerator 2 is also driven by the engine 1 to operate as a generator togenerate electric power and to perform a regenerative braking operation.The driving force/power generation of the motor generator 2 iscontrolled by an inverter 3.

[0029] The inverter 3 is connected on its DC side to an electrolyticcapacitor 4 for ripple elimination. A capacitor 5 is connected inparallel with the electrolytic capacitor 4. The capacitor 5 has a largecapacity and is generally called a capacity of an electrical doublelayer or a supper or ultra capacitor. The equipment also comprises avoltage sensor 8A that senses a voltage across the capacitor 5 and acurrent sensor 9 that senses a current flowing through the capacitor 5.The voltage and current sensed by the voltage sensor 8A and the currentsensor 9 are delivered to a power supply controller 10.

[0030] The capacitor 5 is connected in parallel with a series circuit ofa switching unit 11 and a battery 6. The battery 6 comprises a lead acidbattery of a rated voltage of 36 V. In addition to the lead acidbattery, for example, a high performance battery such as anickel-hydrogen battery or a lithium ion battery is usable. A voltagesensor 8B that senses the voltage of the battery 6 is provided. Thesensed voltage is delivered to the power supply controller 10.

[0031] In the illustrated embodiment, the switching unit 11 comprises aparallel circuit of a MOSFET and a diode D1 embed therein. In theembodiment, the diode D1 is disposed in such a direction that electricalcharging is allowed normally from the inverter 3 to the battery 6. Thus,the switching unit 11 turns on/off only the discharge current flowingfrom the battery 6 to the inverter 3.

[0032] The switching unit 11 is connected in parallel with a seriescircuit of a resistor 13 and another switching unit 12. In theillustrated embodiment, the switching unit 12 comprises a parallelcircuit of a MOSFET and a diode D2. When a difference in voltage betweenthe capacitor 5 and the battery 6 is large and a large current may flowthrough the battery 6 if the capacitor 5 and the battery 6 are directlyconnected by the switching unit 11, the switching unit 12 is renderedconductive to thereby cause an electrical current to flow through theresistor 13. This causes the value of the electric current flowingthrough the battery 6 to decrease and to restrict a flow of an excessivecurrent through the battery.

[0033] Generally, a plurality of 42-V loads 7 is connected to thebattery 6. A DC/DC converter (not shown) is connected to the battery 6to supply the 14-V system with power.

[0034] The power supply controller 10 receives information on thevoltage/current of the capacitor 5, the voltage of the battery 6, andthe current/rotational speed of the motor generator 2 and gives controlcommands to the inverter 3 and the switching units 11 and 12 at avehicle controller's request to start up and assist in accelerating theengine and to perform a regenerative breaking operation.

[0035] As described above, the characteristic composition of the presentembodiment is that the capacitor 5 is connected at all times on the DCside of the inverter 3 and that the battery 6 is connected through theswitching units 11 and 12 to the capacitor 5. Thus, the battery 6 isconnected to and disconnected from the DC side of the inverter 3 byturning on and off the switching units 11 and 12. Such circuitcomposition is expressed as “The capacitor of an electrical double layeris directly connected to the DC side of the inverter and the battery isconnected in parallel with the capacitor of an electrical double layervia the first switching unit” in the present embodiment.” The “direct”connection of the capacitor of an electrical double layer to the DC sideof the inverter means that no a switching unit is connected in a line onthe side of the capacitor 5 in terms of the parallel circuit of thecapacitor 5 and the battery 6. Thus, note that a circuit composition inwhich a switching unit such as a contactor provided on a line betweenthe DC side of the inverter 3 and the junction point of the switchingunit 11 and the capacitor 5 is turned off to disconnect both thecapacitor 5 and the battery 6 from the DC side of the inverter 3 shouldfall within the scope of the composition expressed by the wording “Theelectrical double layer capacitor is directly connected to the DC sideof the inverter and the battery is connected in parallel with theelectrical double layer capacitor via the first switching unit” in thepresent embodiment.

[0036] Now, referring to FIG. 2, control of the power supply controller10 for use in the power supply equipment for the motor vehicle accordingto the present embodiment will be described next. FIG. 2 illustrates aflow of control from the start-up of the engine to the normal powergeneration.

[0037] The engine is basically started up only with power stored in thecapacitor 5. Thus, in step S101 the power supply controller 10 turns offthe switching unit 11 and 12. In the example of FIG. 2, the switchingunits 11 and 12 are indicated as SW 1 and SW 2, respectively. By turningoff the switching units 11 and 12, no electrical current flows from thebattery 6 to the inverter 3. Thus, the engine can be started up onlywith electric power stored in the capacitor 5.

[0038] Then, in step S102 the power supply controller 10 senses avoltage Vc across the capacitor 4 with the voltage sensor 8A anddetermines whether the voltage Vc is sufficient to start up the engine.It is now assumed that a threshold value is set at 30 V, for example. Ifthe voltage Vc is not lower than the threshold value, the control passesdirectly to step S109 whereas if the voltage Vc is lower than thethreshold value, the control passes to step S103. If the voltage Vc isnot lower than the threshold value, in step S109 the engine is startedup. If the voltage across the capacitor 5 is lower than the thresholdvalue due to discharge of the capacitor 5, for example, when the vehicleis left alone for a long time, the battery 6 charges the capacitor 5 insteps S103 and subsequent steps.

[0039] When the voltage Vc across the capacitor 4 is lower than thethreshold value, in step S103 the power supply controller 10 comparesthe capacitor voltage Vc with the battery voltage Vb sensed by thevoltage sensor 8B. In order to determine whether a differential (Vb−Vc)between the battery voltage Vb and the capacitor voltage Vc is small ornot, a voltage of 10 V, for example, is set as a second threshold value.If the differential is small, for example, not more than 10 V, the powersupply controller 10 turns on the switching unit 11 in step S104 anddirectly charges the capacitor 5 from the battery 6 in step S106.

[0040] If the voltage difference is large, for example, more than 10 V,the switching unit 12 is turned on in step S105 so that the battery 6 isconnected to the capacitor 5 via the resistor 13 and that the battery 6charges the capacitor 5 in S106, in order to avoid a flow of a largecurrent occurring through the switching unit 11 when the battery 6 isconnected directly to the capacitor 5. If the resistor has, for example2 Ω, the discharging current can be restricted to 10 A even when thevoltage difference is 20 V.

[0041] Then, in step S107 the power supply controller 10 determines acharging state of the capacitor 5. The voltage across the capacitor 5includes a voltage rise (r·I) occurring due to its internal resistance rwhen the capacitor 5 is charged. Thus, it is necessary to determine thevoltage across the capacitor 5 by subtracting the voltage rise from thecharging voltage where I is a value of an electrical current flowingthrough the capacitor 5 and sensed by the current sensor 9. In step S107it is determined whether the capacitor 5 has stored sufficient energydue to charging depending on whether (Vc−r·I) has increased, forexample, to not lower than 30 V. The capacitor 5 continues to be chargedin step S106 until the capacitor is sufficiently charged, at which timethe control passes to step S108 where the power supply controller 10turns off both the switching units 11 and 12.

[0042] If the determination in step S102 is yes or the processing instep S108 is terminated, in step S109 the power supply controller 10gives a drive command to the inverter 3 to drive the motor generator 2and start up the engine 1.

[0043] After the engine 1 is started up, in step S110 the motorgenerator 2 operates as a generator and the power supply controller 10controls the inverter 3 to thereby charge the capacitor 5. Sincestarting up the engine 1 consumes the energy stored in the capacitor 5,the voltage across the capacitor 5 lowers temporarily. However, theoperational mode of the motor generator 2 switches to a generator mode,and at the capacitor voltage rises.

[0044] Then, in step S111 the power supply controller 10 checks thevoltage across the capacitor 5. More specifically, the power supplycontroller 10 monitors a difference (Vb−Vc) between the voltage Vb ofthe battery 6 and the voltage Vc across the capacitor 5. The powersupply controller 10 continues to charge the capacitor 5 in step S110until the voltage difference becomes not more than 2 V, for example, atwhich time the power supply controller 10 turns off the switching unit11 to connect the capacitor 5 to the battery 6 to thereby charge thebattery 6.

[0045] In the generator mode, the capacitor 5 and the battery 6 areoperated in parallel connection. Even when the switching unit 11 is notturned on, supply of power to the battery 6 is possible due to operationof a built-in diode D1 formed to be connected in parallel with answitching element of the switching unit 11. By turning on the switchingunit 11, the on-resistance and hence an on-loss produced at all timesare reduced.

[0046] While in the embodiment the series circuit of the resistor 13 andthe switching unit 12 is used, this series circuit may be omitted whenthe internal resistance values of the capacitor 5 and the battery 6 arelarge or a transitional large current is allowed to flow through thoseelements.

[0047] While in the embodiment the rated voltage of the power supply isassumed as 42 V, the present invention is applicable when the ratedvoltage of the power supply is another voltage such as 14 or 28 V.

[0048] As described above, in the present embodiment when the engine 1is started up, the power stored in the capacitor 5 is used and thebattery 6 is separated from the capacitor 5 by turning off the switchingunits 11 and 12. The capacitor 5 generally has a small internalresistance compared to the battery and a voltage drop across thecapacitor 5 is small even when a large current discharges through thecapacitor 5. The capacitor 5 has a good temperature characteristic and asmall reduction in its output compared to the battery even at lowtemperatures such as −30° C. Thus, as indicated by the presentembodiment, the use of the capacitor ensures the start-up of the engine.

[0049] When the charge rate of the battery lowers in the prior art, theoutput of the battery lowers, so that the engine may not be started upsatisfactorily. However, in the present embodiment the engine can bestarted up surely as long as the power sufficient to charge thecapacitor remains in the battery. Thus, the capacity of the battery canbe reduced. Since a large current does not flow into the battery at thestart-up of the engine, a deterioration in the battery is prevented.

[0050] In the generator mode, the capacitor 5 and the battery 6 areoperated in parallel connection, so that a deterioration in the battery6 is prevented. More particularly, since the capacitor 5 has smallinternal resistance compared to the battery 6, the capacitor 5 shares alarger current compared to battery 6 when the capacitor 5 and thebattery 6 are operated in the parallel connection. Thus, even when alarge charging current flows through the parallel circuit of thecapacitor 5 and the battery 6 in the regenerative breaking operation,the current flowing through the battery 6 is reduced to thereby suppressa deterioration in the battery 6. Since a large regenerative current isallowed to flow through the parallel circuit of the capacitor 5 andbattery 6 compared to the use of the battery alone, more regenerativepower can be absorbed and the absorbed energy can be used to assist inthe acceleration of the vehicle and to drive an auxiliary machine tothereby improve the fuel economy of the motor vehicle.

[0051] Since current ripples produced due to the switching operation ofthe inverter 3 are absorbed by the capacitor 5, the filter capacitor 4can be omitted or reduced in capacity. When the distance between theinverter 3 and the capacitor 5 is large, an inductance involved in thelines extended between the inverter 3 and the capacitor 5 is influencedor increased. Thus, in this case the filter capacitor 4 should beprovided on the inverter side. The capacity of the filter capacitor 4may be small in consideration of the case where no capacitor 5 isprovided.

[0052] According to the composition of the present embodiment, even whenthe battery 6 goes flat, the engine can be jump-started up from a 14-Vpower supply installed in another vehicle. Since the battery 6 isseparated from the inverter 3 by the switching units 11 and 12, a 14-Vpower supply installed in another vehicle can be connected to theinverter 3 to charge the capacitor 5. When the voltage across thecapacitor 5 rises to about 14 V, the inverter 3 is operated and hencethe motor generator 2 is driven to start up the engine 1. In this case,it is necessary to design beforehand the motor generator 2 so that themotor generator 2 can start up the engine even at 14 V.

[0053] As described above, according to the present embodiment, low-costpower supply equipment for a motor vehicle is obtained that prevents adeterioration in the battery to thereby give the battery a long servicelife; that reduces the capacity of the electrolytic capacitor; and thateliminates the DC/DC converter.

[0054] Next, with reference to FIG. 3, power supply equipment for amotor vehicle according to a second embodiment of the prevent inventionwill be described.

[0055]FIG. 3 is a block diagram of the power supply equipment for amotor vehicle according to the second embodiment. The same referencenumeral in FIGS. 3 and 1 denotes the same component of the equipment.

[0056] In the present embodiment, diodes D1 and D2 in switching units11A and 12A are reverse in direction to those in the switching units 11and 12 of FIG. 1, so that an electrical current flowing from the battery6 to the inverter 3, or a discharge current, flows at all times. In thiscase, when the motor generator 2 acts as a driver to start up/assist inaccelerating the engine, the capacitor 5 and the battery 6 are usedtogether. While the discharge current is supplied from both thecapacitor 5 and the battery 6, the current flowing through the capacitor5 is large compared to that flowing through the battery 6 because theinternal resistance of the capacitor 5 is smaller than that of thebattery to thereby restrict a deterioration in the battery 6. Inaddition, the start up of the engine at low temperatures is improved.

[0057] In the generator mode, the power supply controller 10A normallyturns on the switching unit 11A to cause the capacitor 5 and the battery6 to operate together. In the regenerative braking operation where alarger charging current flows in the equipment, the switching unit 11Ais turned off so that only the capacitor 5 absorbs the energy. The powersupply controller 10A causes the switching unit 11A to cooperate withthe inverter 3 so that when the charging current exceeds a thresholdvalue, the switching unit 11A is turned off.

[0058] In order to enable the battery to be charged with a largecurrent, its capacity should increase and its electrodes should beespecially devised. However, according to the present embodiment, thenecessity for charging the battery with a large current is eliminated sothat the cost is reduced. In addition, a deterioration in the equipmentis prevented to thereby increase its service life.

[0059] Even when the voltage across the capacitor 5 increases greatlydue to a large current flowing therethrough, a mode can be selected inwhich the switching unit 12A connected to the battery 6 is turned on tothereby prevent a large excessive current from flowing through thebattery and hence to use the battery and the capacitor together.

[0060] As described above, according to the present embodiment, low-costpower supply equipment for a motor vehicle is obtained that prevents adeterioration in the battery to thereby increase its service life; thatreduces the capacity of the electrolytic capacitor; and that eliminatesthe necessity for the DC/DC converter.

[0061] Next, referring to FIG. 4, a composition of power supplyequipment for a motor vehicle according to a third embodiment of thepresent invention will be described.

[0062]FIG. 4 is a block diagram of the power supply equipment for amotor vehicle according to the third embodiment. The same referencenumeral denotes the same or similar component in FIGS. 4 and 1.

[0063] In the present embodiment the switching units 15 and 16 are usedto turn on and off bi-directional currents whereas the switching units11 and 12 of FIG. 1 and the switching units 11A and 12A of FIG. 3control only the unidirectional currents. More specifically, theswitching units 15 and 16 each use a pair of reversely parallelconnected thyristors to switch on/off a current flowing in bothdirections therethrough. In place of the pair of reverselyparallel-connected thyristors, a pair of reversely series-connectedMOSFETs may be used to fulfill a similar function.

[0064] The present embodiment produces advantageous effects equal tocombined advantageous effects produced by both the embodiments of FIGS.1 and 3. In each of the engine start-up and the regenerative breakingoperation, the power supply controller 10B can turn off the switchingunits 15 and 16 so that only the capacitor 5 charges/discharges thebattery 6. Thus, the charging and discharging electric currents flowingthrough the battery 6 can be restricted to small currents to therebyreduce the cost and increase its service life.

[0065] As described above, according to the present embodiment, thepower supply equipment for a motor vehicle is obtained which prevents adeterioration in the battery to thereby increase its service life, whichreduces the capacity of the electrolytic capacitor, and which eliminatesthe DC/DC converter to thereby reduce the cost.

[0066] A composition of power supply equipment for a motor vehicleaccording to a fourth embodiment of the present invention will bedescribed next with respect to FIG. 5, which is a block diagram of thepower supply equipment. The same reference numeral denotes the same orsimilar component in FIGS. 5 and 1.

[0067] In the present embodiment the power supply equipment for a motorvehicle uses a battery 6A with a plurality of (3) voltage terminals;that is, 42- and 14-V terminals and a grounded terminal.

[0068] The 42- and 14-V terminals of the battery 6A are connected viaswitching units 17 and 18, respectively, to an inverter 3. When theswitching units 17 and 18 each comprise a MOSFET, built-in diodes D3 andD4 are formed in reverse directions to the 42- and 4-V terminals of thebattery 6A, as shown, to thereby avoid short-circuiting the 42- and 14-Vterminals. In this embodiment, a plurality of 14-V loads 7 a isconnected across the 14-V terminal of the battery 6A and ground.

[0069] The power supply controller 10C controls the turning on/off ofthe switching units 17 and 18 to switch the inverter 3 to either a 42-or 14-V operation. In the 42-V operation the switching units 17 and 18are turned on and off, respectively, whereas in the 14-V operation theswitching units 17 and 18 are reversely turned off and on, respectively.

[0070] When a large current flows through the circuit concerned, forexample, as in the engine startup, regenerative braking operation oracceleration assistance or the like, the 42-V operation is basicallyperformed. The connection of the components on the 42-V terminal side issimilar to that of the first embodiment of FIG. 1, and hence theoperation of the power supply equipment ranging from the start-up of theengine to the power generation should be performed in a process similarto that shown in FIG. 2. While in the present embodiment an element suchas the resistor 13 of FIG. 1 is not shown, such element can be connectedin position as shown in FIG. 1, if necessary.

[0071] The power supply equipment is operated ordinarily at 42 V even inthe generator mode whereas the power supply controller 10C turns off andon the switching units 17 and 18, respectively, so that the battery 6Ais charged only on its 14-V terminal side to thereby eliminate anunbalance in charge rate between the 42 and 14-V terminal sides thereof.

[0072] According to the present embodiment, use of the 3-terminalbattery brings about a low-cost dual (42, 14-) volt power supply system.Additional use of the capacitor serves to reduce the capacity of thebattery and to increase its service life.

What is claimed is:
 1. Power supply equipment for a motor vehicle,comprising a motor generator, an inverter for driving said motorgenerator, a battery and a capacitor of an electrical double layer,wherein said capacitor is directly connected to a DC side of saidinverter and said battery is connected in parallel with said capacitorvia first switching means.
 2. The power supply equipment for a motorvehicle according to claim 1, further comprising: control means forturning off said first switching means in the start-up of an engine toseparate said battery from said capacitor and for turning on said firstswitching means after the start-up of the engine to connect said batteryto said capacitor.
 3. The power supply equipment for a motor vehicleaccording to claim 2, further comprising: a series circuit of a resistorand second switching means being connected in parallel with said firstswitching means.
 4. The power supply equipment for a motor vehicleaccording to claim 3, wherein: said control means determines which ofsaid first and second switching means should be turned on, depending ona difference in voltage between said capacitor and said battery whensaid battery and said capacitor are connected by said first or secondswitching means.
 5. The power supply equipment for a motor vehicleaccording to claim 1, further comprising: a second capacitor connectedin parallel with said first-mentioned capacitor between said inverterand said capacitor for eliminating high frequency ripples.
 6. The powersupply equipment for a motor vehicle according to claim 1, wherein: saidfirst switching means comprises a diode conductible at all times so asto allow an electrical current to flow from said inverter to saidbattery.
 7. The power supply equipment for a motor vehicle according toclaim 1, wherein: said first switching means comprises a diodeconductible at all times so as to allow an electrical current to flowfrom said battery to said inverter.
 8. Power supply equipment for amotor vehicle, comprising a motor generator, an inverter for drivingsaid motor generator, a battery and a capacitor of an electrical doublelayer, wherein said battery has a plurality of different (higher andlower) voltage terminals; said capacitor being directly connected to aDC side of said inverter; said capacitor being connected on its highervoltage side to a higher voltage terminal of said battery via firstswitching means; and said capacitor being connected on its highervoltage side to a lower voltage terminal of said battery via secondswitching means.